1) Field of the Invention
The present invention relates to an information recording medium, where information is recorded by laser beam irradiation, and a method for manufacturing the information recording medium, and particularly to optical discs for overwriting such as a phase change optical disc, e.g. DVD-RAM, DVD-RW, etc., magneto-optical discs, e.g. MD, MO, etc., and a write-once optical disc, e.g. DVD-R, etc., and a method for manufacturing the same.
The information recording medium may be referred to hereinafter as xe2x80x9cphase change optical discxe2x80x9d or merely by xe2x80x9coptical discxe2x80x9d, but in the present invention any information recording medium can be used, so far as it generates heat by laser beam irradiation and the atomic arrangement or magnetic moment is changed by the generated heat to record information. Particularly, other information mediums than disc-type information recording mediums such as an optical card, etc. can be used, irrespective of the shapes of aninformation recording medium.
Laser beam may be referred to hereinafter merely by xe2x80x9claser lightxe2x80x9d or merely xe2x80x9clightxe2x80x9d, but as mentioned above, any laser beam is effective for the present invention, so far as it can generate heat on the information recording medium and can give a multiplex interference effect by a plurality of interference layers having different refractive indice. The present invention has been established by use of red laser (wavelength: 645-660 nm), but the present invention is not particularly limited to laser wavelength, but the present invention can be effectively applied to high density optical discs using a laser of relatively short wavelength such as blue laser, ultraviolet laser, etc.
2) Description of Related Art
Recently, phase change optical discs such as 2.6 GBDVD-RAM, etc. have been commercialized on the basis of such a feature as distinguished reproduction interchangeability with read-only optical discs such as DVD-ROM, DVD-Video, etc. However, 2.6 GBDVD-RAM is not satisfactory for a consumer""s request for the recording capacity. There is an increasing demand for 4.7 BGDVD-RAM OR 4.7 GBDVD-RW. That is, the phase change optical discs have the same recording capacity as that of DVD-Video, so that VTR may be replaced with optical discs for video recording.
For practical use of 4.7 GBDVD-RAM, there are many problems to be solved. Conventional methods so far proposed for solving these problems will be described in detail below.
When the track pitch of an information recording medium is narrowed to about 80% of laser beam spot, generally leak-in of read signals from the adjacent information recording tracks occurs. Leak-in of read signals from information recorded in the adjacent information recording tracks is called xe2x80x9ccross-talkxe2x80x9d. To solve this cross-talk problem, a land-groove recording system has been developed, as will be described below.
The plastic substrate of an optical disc for overwriting is in a concave-convex shape (groove shape) for tracking a laser beam, and it is usual to record information on the concave parts or on the convex parts. Recently, to improve the recording density (to narrow the track pitch), a method for individually recording information on the concave parts and on the convex parts on the basis of the concave-convex shape of the plastic substrate was developed.
The convex parts and the concave parts of the concave-convex shaped substrate will be hereinafter referred to as xe2x80x9clandsxe2x80x9d and xe2x80x9cgroovesxe2x80x9d, respectively. When the track pitch of an information recording medium is narrowed to about 80% of laser beam spot and information is recorded both on lands and in grooves, leak-in of read signals from adjacent information medium tracks (grooves in contrast to lands or lands in contrast to grooves) generally occurs. For example, when the information recorded on lands is to read, such problems as leak-in of read signals from the information recorded on grooves, failure to exactly read the information recorded on lands, etc. will be encountered. In the land-groove recording system, the cross-talk problem has been solved by setting the groove depth to xcex/7xe2x88x92xcex/5 (xcex: laser wavelength) (Prior Art 1: JP-A-6-338064). The feature of this system is that even in the case of narrowing the track pitch to about 60% of laser beam spot, cross-talk from the adjacent information recording tracks (leak-in of signals from the adjacent information recording tracks) can be cancelled.
Prior art 1 is a distinguished method, but any means for controlling a phenomenon of erasing record marks on the adjacent information tracks (adjacent grooves when recorded on lands or adjacent lands when recorded in grooves) i.e. so called a cross-erase phenomenon, has not been fully taken into consideration. For example, in Prior Art 1, the distance between the recording layer and the heat diffusion layer (reflective layer) is as small as 18 nm, so that heat is diffused into the adjacent information recording tracks through the heat diffusion layer at the time of recording information. Thus, such a problem as easy occurrence of a cross-erase phenomenon is encountered (Problem 1).
On the other hand, a method for improving the reflectance by providing three interference layers having different reflectances, e.g. ZnSxe2x80x94SiO2 layer and SiO2 layer, on the laser beam incident side of the recording layer, thereby attaining a multiplex interference effect is known from No. 5 Phase Change Study Group, symposium Lecture Manuscripts, pp 9-14, 1993 (Prior Art 2). In the method of Prior Art 2, the ZnSxe2x80x94SiO2 layer is in contact with the recording layer, and thus when several thousand runs of overwriting are carried out sulfur (S) elements in the ZnSxe2x80x94SiO2 layer are diffused into the recording layer, thereby lowering the reflectance (Problem 2).
To prevent lowering of the reflectance due to such several thousand runs of overwriting, a method for providing an interface layer composed of a dielectric compound with a high melting point such as SiO2, Al2O3, etc. between the recording layer and the ZnSxe2x80x94SiO2 dielectric protective layer is well known from JP-A-10-228676 (Prior Art 3).
However, when the present inventors carried out high temperature humid tests, peeling took place between the recording layer and the interface layer, and the present inventors have found that the method of Prior Art 3 is impractical (Problem 3).
In the case of combination of Prior Art 2 with Prior Art 3, there will be a total of 4 thin layers on the laser beam incident side of the recording layer, and the increased number of layers is manufacturing-wise not preferable. In Prior Arts 1, 2 and 3, occurrence of a cross-erase phenomenon is not fully taken into consideration, and in the case of narrowing the track pitch a cross-erase phenomenon occurs, depending on the thickness of the individual layers (the same problem as Problem 1).
To solve said Problems 1 to 3, the present inventors have studied improvement of the structure of an information recording medium but have encountered further Problems 4 to 11, as will be described later.
An object of the present invention is to provide an information recording medium free from all the Problems 1 to 11.
Another object of the present invention is to provide a method for manufacturing an information recording medium at a low cost.